The miniaturization of device components has become one of the key driving principles in enabling overall device scaling and, particularly, in enabling the production of increasingly compact devices with greater performance, reduced power consumption, and lower production costs. Advances in photolithography have, in part, enabled continued device scaling. Currently, extreme ultraviolet (EUV) photolithography is poised to complement and eventually replace conventional deep ultraviolet (DUV) photolithography due to the significantly narrower illumination wavelength used, providing enhanced patterning resolution and lower process complexity, among other benefits. For example, depending upon the platform used and the technology node at issue (e.g., the 7 nm technology node), EUV photolithography techniques employing an EUV light with a wavelength (λ) of 13.5 nm may be used to achieve a less than 10 nm half pitch resolution at a single exposure, whereas DUV photolithography employs a DUV light with a wavelength (λ) of 193 nm in order to achieve a minimum 40 nm half pitch resolution at single exposure.
Due to the considerable absorption of EUV radiation by all forms of matter, the optical elements used in EUV photolithography are based completely on reflective rather than refractive optics. Specifically, EUV photolithography techniques use a unique photomask structure. This photomask structure typically includes a substrate; a multilayer stack on the substrate; and a light absorber layer on the multilayer stack. The substrate can be formed of a low thermal expansion material (LTEM) (i.e., the substrate can be a LTEM substrate). The multilayer stack can be formed of alternating layers of high and low atomic number materials (i.e., EUV mirrors), which form a Bragg reflector for guiding and shaping EUV photons, and a protective layer above the alternating layers to prevent material degradation. The light absorber layer can be formed of a material that absorbs EUV light.
During EUV photolithography, the EUV light absorber layer of the photomask structure is patterned and etched to represent desired circuit features and can be subsequently exposed to EUV light. The EUV light is reflected off the photomask structure and directed so as to expose a target and, particularly, so as to expose a photosensitive layer above a feature layer to be patterned. By exposing the photosensitive layer to EUV light reflected off the photomask structure, the pattern of the etched light absorber layer is transferred into the photosensitive layer. It should be noted that the LTEM substrate prevents distortion of the reflected light due to heating of the photomask structure during exposure. In any case, after the pattern is transferred into the photosensitive layer, it can subsequently be transferred into the feature layer below.
Unfortunately, this reflective photomask design introduces a new class of defects not seen in previous mask technologies. Specifically, particles can become embedded in the multilayer stack during its formation (e.g., during thin film deposition) and these particles present as defects in the EUV mirrors of the Bragg reflector. Such defects can function as light absorbers, rather than being reflective, and can, thus, result in errors in the pattern transferred into the photosensitive layer and, subsequently, into the feature layer. In other words, such defects can negatively impact image formation and printing. Therefore, EUV photomask defectivity is a persistent obstacle that must be addressed in order to enable high volume manufacturing (HVM).